Palladium and platinum-based nanoparticle functional sensor layers for selective H2 sensing

ABSTRACT

The disclosure relates to a plasmon resonance-based method for H 2  sensing in a gas stream utilizing a hydrogen sensing material. The hydrogen sensing material is comprises Pd-based or Pt-based nanoparticles having an average nanoparticle diameter of less than about 100 nanometers dispersed in an inert matrix having a bandgap greater than or equal to 5 eV, and an oxygen ion conductivity less than approximately 10 −7  S/cm at a temperature of 700° C. Exemplary inert matrix materials include SiO 2 , Al 2 O 3 , and Si 3 N 4  as well as modifications to modify the effective refractive indices through combinations and/or doping of such materials. The hydrogen sensing material utilized in the method of this disclosure may be prepared using means known in the art for the production of nanoparticles dispersed within a supporting matrix including sol-gel based wet chemistry techniques, impregnation techniques, implantation techniques, sputtering techniques, and others.

RELATION TO OTHER APPLICATIONS

This patent application is a continuation-in-part of and claims priority from provisional patent application 62/065,964 filed Oct. 20, 2014, which is hereby incorporated by reference.

GOVERNMENT INTERESTS

The United States Government has rights in this invention pursuant to the employer-employee relationship of the Government to the inventors as U.S. Department of Energy employees and site-support contractors at the National Energy Technology Laboratory.

FIELD OF THE INVENTION

One or more embodiments relates to a method for sensing the H₂ concentration of a gaseous stream through evaluation of the optical signal of a hydrogen sensing material comprised of Pd- or Pt-based nanoparticles dispersed in a matrix material.

BACKGROUND

The ability to selectively sense H₂ is critically important for a broad range of applications spanning energy, defense, aviation, and aerospace. One of the most significant needs is for sensors that are capable of leak detection of H₂ at levels up to the lower explosive limit (˜4% in ambient air). A large body of work therefore exists focused on the research and development of sensors for H₂ leak detection and safety applications with a number of commercial products available on the market. Additional applications of hydrogen sensors requiring operation at elevated temperatures include monitoring of hydrogen in metallurgical processes as well as monitoring the composition of fuel gas streams in power generation technologies such as gas turbines and solid oxide fuel cells. Measurements of H₂ levels dissolved in transformer oil can also enable condition-based monitoring to provide early detection of potential failures with large associated economic and environmental impacts. A broad range of sensor devices and technologies have been applied to hydrogen sensing including chemi-resistive, electrochemical, catalytic, work function, acoustic, and optical-based approaches.

Optical-based sensors are particularly well-suited for H₂ sensing due to a number of inherent advantages that include elimination of electrical wiring and contacts at the sensing location, which provides benefits in terms of safety and sensor longevity in potentially explosive atmospheres, harsh environments, and at high temperatures. A large number of optical-based hydrogen sensor devices have been developed and demonstrated, many of which are based upon the changes in optical properties of a functional sensor material. Pd and Pd-alloy thin films are the most common materials employed for optical H₂ sensing applications due to a well-known dependence of the optical constants of Pd on ambient H₂ concentration. See e.g., Vargas et al., “Optical and electrical properties of hydrided palladium thin films studied by an inversion approach from transmittance measurements,” Thin Solid Films 496 (2006); see also Silva et al., “A Review of Palladium-Based Fiber-Optic Sensors for Molecular Hydrogen Detection,” IEEE Sensors Journal, 12(1) (2012); see also Yang et al., “Fiber Optic Hydrogen Sensors: a Review,” Phototonic Sensors 4(4) (2014), among others. These measurable dependences arise from a large solid solubility of hydrogen ions in the Pd lattice resulting in (1) volume expansion, (2) modifications to free carrier concentration, and (3) alterations to electronic band structure. The overall result is a decrease in the magnitude of the real and imaginary parts of the dielectric constant with increasing H₂ in the ambient atmosphere. Pd thin films have also been used in conjunction with optical fibers in an evanescent wave absorption spectroscopy based approach allowing for direct monitoring of changes in the real and imaginary parts of the refractive index. See e.g., Luna-Moreno et al., “Optical fiber hydrogen sensor based on core diameter mismatch and annealed Pd—Au thin films,” Sensors and Actuators B 125 (2007); see also Wei et al., “Nano-structured Pd-long period fiber gratings integrated optical sensor for hydrogen detection,” Sensors and Actuators B 134 (2008), among others. In many cases, Pd alloys have shown advantageous sensing properties relative to elemental Pd due to improved film morphology stability and a reduction in (1) hysteresis, (2) response time, and (3) interference due to other chemical species present by suppressing a phase transformation to the PdH_(x) phase and also tailoring the surface chemistry. See e.g., Luna-Moreno et al, “Tailored Pd—Au layer produced by conventional evaporation process for hydrogen sensing,” Optics and Lasers in Engineering 49 (2011), among others.

In more recent work, it has been demonstrated that Pd or Pd-alloy nanoparticles on the surface of silica substrates or unclad optical fibers can also be utilized for optical H₂ sensing. For example, AuPd alloy nanoparticles deposited on single-mode fibers mechanically-thinned to 5- or 10-microns in diameter have shown rapid and monotonic responses to H₂ at levels up to the lower explosive limit. See Monzon-Hernandez et al., “Optical microfibers decorated with PdAu nanoparticles for fast hydrogen sensing,” Sensors and Actuators B 151 (2010). In this case, the sensing response was attributed to changes in the effective refractive indices of the particles resulting in a H₂-concentration dependent light scattering that increased with increasing H₂ concentration at an interrogation wavelength of 1550 nm. Pd nanoparticles on the surface of silica glass substrates have also been synthesized through dewetting of continuous films by high temperature annealing and found to respond optically to H₂ when measured in a transmission geometry. See Kracker et al., “Optical hydrogen sensing with modified Pd-layers: A kinetic study of roughened layers and dewetted nanoparticle films,” Sensors and Actuators B 197 (2014). In addition to reversible H₂ responses that are presumably associated with modifications to the optical constants of Pd due to H atoms dissolved into the Pd lattice, this work also demonstrated an irreversible change in transmission upon the first H₂ exposure that was claimed to be due to reduction of the Pd-nanoparticles oxidized during high temperature annealing. Pd nanoparticle-based H₂ sensing layers have a number of potential advantages as compared to continuous thin films of Pd such as (1) response time, (2) sensitivity, and (3) stability of the microstructure at high temperatures or during H₂ loading and unloading cycles. Promising early results have been obtained for such systems but additional work is required to more fully understand the mechanistic origin of the sensing response and to explore the effects of elevated temperatures and the presence of other common gas species previously reported to impact H₂ sensing responses of Pd-based thin films such as O₂ and CO.

The disclosure herein provides a hydrogen sensing material comprising a plurality of Pd-based and/or Pt-based particles dispersed in an inert matrix, where the inert matrix has a bandgap greater than or equal to 5 eV and has an oxygen ion conductivity of less than 10⁻⁷ S/cm at a temperature of 700° C. An exemplary material consists of silica, alumina, a mixture of the two (i.e. aluminosilicate) and the corresponding silica and alumina based nitrides. The hydrogen sensing material provides inherent advantages including: (1) an inherent filtering function of the inert matrix (particularly for amorphous matrices such as silica) allowing for potentially improved H₂ selectivity and minimization of cross-sensitivity to other species such as CO, (2) the chemically inert nature of the matrix (e.g. silica) as a protective layer for the embedded nanoparticles making them suitable for applications in harsh environments, (3) a relatively low refractive index of many exemplary matrix materials (e.g. silica, alumina) which is similar to that of the optical fiber core material allowing enhanced compatibility with waveguide-based sensing devices, and (4) the ability to controllably tune the thickness of the nanocomposite layers for sensor response and device optimization.

These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.

SUMMARY

The disclosure provides a method for H₂ sensing in a gas stream by utilizing the shifts in an optical signal generated by a hydrogen sensing material, where the hydrogen sensing material is comprised of a plurality of Pd-based nanoparticles, Pt-based nanoparticles, or a combination thereof dispersed in an inert matrix. The hydrogen sensing material is in contact with gaseous constituents comprising the gas stream and periodically comprised of diatomic hydrogen H₂, having a concentration which may vary over time. The optical signal is based on a comparison of incident light illuminating the hydrogen sensing material and exiting light which is transmitted, reflected, scattered or a combination thereof by the hydrogen sensing material.

The hydrogen sensing material is comprised of Pd-based and/or Pt-based nanoparticles having an average nanoparticle diameter of less than about 100 nanometers. The Pd-based nanoparticles comprise palladium (Pd), a palladium alloy, or combinations thereof. In an embodiment, the palladium alloy is a palladium-gold alloy, a palladium-silver alloy, a palladium-copper alloy, a palladium-platinum alloy, or combinations thereof. Similarly, Pt-based nanoparticles comprise platinum (Pt), a platinum alloy, or combinations thereof. In an embodiment, the platinum alloy is a platinum-gold alloy, a platinum-silver alloy, a platinum-copper alloy or combinations thereof. The Pd-based and Pt-based nanoparticles are dispersed in an inert matrix having a bandgap greater than or equal to 5 electron volts (eV), and an oxygen ion conductivity of less than approximately 10⁻⁷ S/cm at a temperature of 700° C. Exemplary inert matrix materials include SiO₂, Al₂O₃, and Si₃N₄ as well as derivatives such as MgF₂ doped SiO₂, and mixtures of SiO₂/Al₂O₃. In certain embodiments, matrix materials are specifically chosen to optimize the effective refractive index of the hydrogen sensing material for use as gas sensitive cladding layers in optical waveguide based sensors. Negligible changes in refractive index and a limited number of free carriers of the inert matrix are expected in response to H₂ such that modifications to the optical characteristics of the Pd-based and Pt-based nanoparticles are dominated by direct interactions between the nanoparticles and the ambient atmosphere. Rather than playing an active role in the gas sensing mechanism, the primary role of the inert matrix is two-fold: (1) to mitigate the coarsening of Pd-based and Pt-based nanoparticles under rigorous high temperature conditions and (2) to tailor the effective refractive index of the nanocomposite thin film for optimized sensing response when integrated with an optical waveguide based sensing platform. In many cases, the matrix phase will also be selected to improve hydrogen selectivity by inhibiting chemical diffusion of species other than hydrogen thereby preventing them from reaching the surface of embedded Pd-based and Pt-based nanoparticles. An additional advantage of the embedding matrix is the controlled tunability of the density of the particles within the nanocomposite sensing layer as well as the thickness of the effective layer. Such tunability allows for greater flexibility in optimization of the sensing element for a broad range of potential optical sensor devices as compared to the de-wetted Pd nanoparticle based sensing layers reported by previous investigators.

The hydrogen sensing material utilized in the method of this disclosure may be prepared using means known in the art for the production of nanoparticles dispersed within a supporting matrix including sol-gel based wet chemistry techniques, impregnation techniques, implantation techniques, sputtering techniques, and others. The sensing material may be deposited as a single monolithic layer or through multi-layered deposition involving a single technique or a combination of several film deposition techniques. The sensing material may also be integrated within a multilayered thin film stack for the purpose of optimizing the overall response of a sensor element.

The novel process and principles of operation are further discussed in the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a methodology for sensing H₂ in a high temperature gas stream using the hydrogen sensing material.

FIG. 2 illustrates a hydrogen sensing material configuration suitable for the detection of H₂ using a waveguide sensor.

FIG. 3 illustrates an optical signal generated by an embodiment of the hydrogen sensing material.

FIG. 4 illustrates an optical signal generated by another embodiment of the hydrogen sensing material.

FIG. 5 illustrates performance of the hydrogen sensing material over a range of H₂ concentrations.

FIG. 6 illustrates performance of the hydrogen sensing material over another range of H₂ concentrations.

FIG. 7 illustrates an optical signal in the presence of H₂, CO, and O₂.

FIG. 8 illustrates an optical signal in the presence of H₂ and CO.

FIG. 9 illustrates an additional embodiment of the hydrogen sensing material.

FIG. 10 illustrates an additional embodiment of the hydrogen sensing material.

FIG. 11 illustrates an instrument using the hydrogen sensing material.

DETAILED DESCRIPTION

The following description is provided to enable any person skilled in the art to use the invention and sets forth the best mode contemplated by the inventor for carrying out the invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the principles of the present invention are defined herein specifically to provide a method for H₂ sensing based on the resulting shifts in an optical signal generated by a hydrogen sensing material comprised of Pd-based nanoparticles, Pt-based nanoparticles, or a combination thereof dispersed in an inert matrix.

The disclosure provides a method for H₂ sensing in a gas stream by utilizing the shifts in the optical signal generated by a particular hydrogen sensing material. The hydrogen sensing material comprises a plurality of Pd-based nanoparticles, Pt-based nanoparticles, or a combination thereof dispersed in an inert matrix. The Pd-based nanoparticles comprise palladium (Pd), a palladium alloy, or combinations thereof. In an embodiment, the palladium alloy is a palladium-gold alloy, a palladium-silver alloy, a palladium-copper alloy, a palladium-platinum alloy, or combinations thereof. Similarly, Pt-based nanoparticles comprise platinum (Pt), a platinum alloy, or combinations thereof. In an embodiment, the platinum alloy is a platinum-gold alloy, a platinum-silver alloy, a platinum-copper alloy or combinations thereof. In an embodiment, a metallic component comprises at least 50 wt. %, preferably at least 70 wt. %, more preferably at least 90 wt. % of the plurality of nanoparticles, where the metallic component consists of one of palladium, platinum, a palladium alloy, a platinum alloy, and combinations thereof. In another embodiment at least 30 wt. % and in a further embodiment at least 50 wt. % of the metallic component consists of palladium, platinum, or combinations thereof. The plurality of Pd and/or Pt-based nanoparticles have an average nanoparticle diameter of less than about 100 nm, and the inert matrix has a bandgap exceeding 5 eV and an oxygen ion conductivity of less than approximately 10⁻⁷ S/cm at a temperature of 700° C. In an embodiment, the plurality of Pd and/or Pt-based nanoparticles have an average nanoparticle diameter of less than about 10 nm.

The basic principles of the method are illustrated at FIG. 1. At FIG. 1, light from light source 102 is directed along an optical fiber 104 and focused by lens 105 producing incident light 106 illuminating hydrogen sensing material 101. Concurrently, exiting light 107 is collected behind the specimen using a probe 108 connected to a spectrophotometer 109. Data generated by spectrophotometer 109 or supporting equipment is processed, and an optical signal is displayed. The optical signal is a comparison of the incident light and the exiting light and indicates the absorption, transmission, reflection, and scattering of the incident light at certain wavelengths by hydrogen sensing material 101. The optical signal generally indicates selective photon absorption at certain wavelengths by hydrogen sensing material 101. The hydrogen sensing material 101 is additionally in contact with a gas stream G. Incident light 106, hydrogen sensing material 101, and exiting light 107 generate an optical signal which depends on the hydrogen concentration of the gas stream G, and the optical signal at monitored wavelengths is indicative of the hydrogen concentration and any changes in the hydrogen concentration.

An additional embodiment is depicted at FIG. 2, where the hydrogen sensing material 201 is illuminated by a wave propagating along a waveguide, such as a fiber optic cable. The waveguide is comprised of a core material 234 in contact with a cladding material 235, where core material 234 has a refractive index greater than cladding material 235. For example, core material 234 and cladding material 235 may be comprised of silica and various additions such as germanium, titanium, phosphorous, boron, fluorine, or other dopants in order to alter the respective refractive indices and meet the necessary criteria. At FIG. 2 light source 202 emits light into core material 234, generating wave 236 penetrating cladding material 235. Additionally at FIG. 2, hydrogen sensing material 201 having the properties disclosed is placed in contact with core material 234 such that hydrogen sensing material 201 is illuminated by wave 236 as incident light, as illustrated. Exiting light 207 is collected by probe 208, connected to spectrophotometer 209. Illumination of hydrogen sensing material 201 by wave 236 enables evaluating the hydrogen concentration of gas stream G in contact with hydrogen sensing material 201 by monitoring a shift in the optical signal, as earlier described. The optical power and penetration depth of wave 236 into cladding 235 and hydrogen sensing material 201 can be described by Beer-Lambert law in many cases. See e.g., Dickinson et al., “Convergent, Self-Encoded Bead Sensor Arrays in the design of an Artificial Nose,” Anal. Chem. 71 (1999), among others. As is similarly understood, the optical power coupled into the evanescent field may be improved by various methods such as bending, optimizing the relative refractive indices of the core and cladding, use of hollow fibers, and other methods. See e.g., Elosua et al., “Volatile Organic Compound Optical Fiber Sensors: A Review,” Sensors 6 (2006), among others. Responses that are substantially absent without fiber bending are typically less preferred due to limitations imposed upon optical fiber sensor design and deployment including an ability to perform distributed interrogation.

As an example, FIG. 3 illustrates the results of the method using an apparatus similar to that depicted at FIG. 2, where the hydrogen sensing material comprises Pd-based nanoparticles and a matrix material silica (Pd/SiO₂), where the hydrogen sensing material is in contact with a gas stream at room temperature. FIG. 3 illustrates the optical signal 310 as normalized transmission as a function of experiment time. Measured H₂ sensing responses are presented for a range of H₂ levels from 2% to 100% by volume in a balance of ultra-high purity (UHP) N₂, with H₂ levels of 2%, 3%, 4%, 10%, 25%, 50%, and 100% over time periods Δt₁, Δt₂, Δt₃, Δt₄, Δt₅, Δt₆, and Δt₇ respectively. The results show that at room temperature, a monotonic response can be observed over the entire range of H₂ levels with sufficiently rapid response and recovery times for practical applications. FIG. 4 illustrates results for a similar PdAu/SiO₂ coated optical fiber, illustrating optical signal 411 as normalized transmission with similar H₂ levels of 2%, 3%, 4%, 10%, 25%, 50%, and 100% over time periods Δt₁, Δt₂, Δt₃, Δt₄, Δt₅, Δt₆, and Δt₇ respectively. Similar results are obtained at elevated gas stream temperatures such as 200° C., 400° C., and 600° C. See Ohodnicki et al., “Pd/SiO₂ and AuPd/SiO₂ nanocomposite-based optical fiber sensors for H₂ sensing applications,” Sensors and Actuators B 214 (2015), which is incorporated by reference.

Within this disclosure, “optical signal” means a comparison of light incident on the H₂ sensing material and light exiting the H₂ sensing material at one or more wavelengths using optical spectroscopy. Correspondingly, the optical signal may reflect one specific wavelength, or may reflect a monitored band of wavelengths. The optical signal may be expressed as, for example, a transmittance at the one or more wavelengths, an absorption at the one or more wavelengths, or any other parameters which indicate the absorption, transmission, reflection, scattering or other optical impacts on the incident light as a result of interaction with the H₂ sensing material. As is understood, optical spectroscopy based on a comparison of the incident light and the exiting light may indicate the absorption, transmission, reflection, scattering, and optical impacts which occur as a result of interaction between the incident light and the H₂ sensing material. See e.g., Ingle, James D., and Stanley R. Crouch, Spectrochemical analysis, Englewood Cliffs, N.J.: Prentice Hall, 1988; see also Sole, Jose, An Introduction to the Optical Spectroscopy of Inorganic Solids (2005); see also Sarid, Dror and Challener, William, Modern Introduction to Surface Plasmons: Theory, Mathematica Modeling, and Applications (2010), among others. Additionally, the optical signal as disclosed here is generally not constrained to a specific wavelength or band of wavelengths. For example, the optical signal may occur at one or more wavelengths typically considered to be ultraviolet, visible, or near-infrared as those terms are used in the art, as well as wavelengths falling outside those delineated ranges.

In prior art, localized surface plasmon resonance (LSRP) based sensing materials have been disclosed such as Au-nanoparticles embedded in an inert matrix of the type disclosed here. See e.g. U.S. Pat. No. 8,741,657 issued to Ohodnicki et al., issued Jun. 3, 2014. For such materials, the chemical sensing response is believed to be associated with a charge transfer between the chemical species to be sensed and the sensing material such that a shift in the LSPR absorption peak of the metallic particles associated with a modified free carrier density of the Au nanoparticles can be resolved. See e.g. Baltrus et al., “Examination of charge transfer in Au/YSZ for high-temperature optical gas sensing,” Applied Surface Science 313 (2014). In the sensing materials disclosed here, the changes in optical response are typically associated with direct absorption of H₂ within the Pd-based and Pt-based nanoparticle lattice resulting in modifications to the effective optical constants of Pd-based and Pt-based nanoparticles. Such mechanisms result in an enhanced selectivity relative to charge transfer-based responses due to the specific affinity of Pd and Pd-based as well as Pt and Pt-based alloys for high levels of H₂ dissociation and absorption within the crystalline lattice. See e.g. Ohodnicki et al., Sensors and Actuators B 214 (2015). In some cases, responses for Pd-based and Pt-based nanoparticles may also be associated with modifications to the oxidation state which is not observed for Au-LSPR based sensing materials due to their relatively high degree of nobility. In cases where it is desired for oxidation to be minimized, Pt-based nanoparticles may be employed due to the higher nobility as compared to Pd-based nanoparticles.

As additional examples, optical signals as a function of H₂ concentration in the gas stream are summarized at FIGS. 5 and 6 for varying temperatures. FIG. 5 illustrates H₂ sensing response for both Pd/SiO₂ and AuPd/SiO₂ materials at lower H₂ concentrations, where optical signals 512, 513, and 514 represent the response for the Pd/SiO₂ material at room temperature, 200° C., and 400° C. respectively versus the H₂ concentrations shown, and optical signals 515, 516, and 517 represent the response for the AuPd/SiO₂ material at room temperature, 200° C., and 400° C. respectively versus the H₂ concentrations shown. Similarly, FIG. 6 illustrates H₂ sensing response for both Pd/SiO₂ and AuPd/SiO₂ materials for H₂ concentrations up to 100%, where optical signals 612, 613, and 614 represent the Pd/SiO₂ material at room temperature, 200° C., and 400° C. respectively, and optical signals 615, 616, and 617 represent the AuPd/SiO₂ material at room temperature, 200° C., and 400° C. respectively.

FIG. 7 illustrates the performance of a Pd/SiO₂ hydrogen sensing material in the presence of H₂, CO, and O₂. At FIG. 7, a 20% O₂ background was maintained over time periods Δt₁, Δt₂, and Δt₃, while H₂ was varied with an absence of CO over time period Δt₁, CO was varied with an absence of H₂ over time period Δt₂, and H₂ was varied in the presence of 1% CO over time period Δt₃. Optical signal 717 illustrates the response of the Pd/SiO₂ hydrogen sensing material at a wavelength of 525 nm. As illustrated, the hydrogen sensing response of the Pd/SiO₂ hydrogen sensing material remains in the simultaneous presence of O2, CO, and H2. As an additional example, FIG. 8 illustrates the performance of the Pd/SiO₂ hydrogen sensing material in the presence of H₂ and CO with optical signal 818, generated while H₂ varied with an absence of CO over time period Δt₁, CO varied with an absence of H₂ over time period Δt₂, and H₂ varied in the presence of 1% CO over time period Δt₃. See Ohodnicki et al., Sensors and Actuators B 214 (2015), which is incorporated herein by reference.

Hydrogen sensing material 101 is comprised of a plurality of Pd-based nanoparticles, Pt-based nanoparticles, and combinations thereof dispersed in an inert matrix, where the Pd-based nanoparticles comprise palladium (Pd), a palladium alloy, or combinations thereof, and where the Pt-based nanoparticles comprise platinum (Pt), a platinum alloy, or combinations thereof. In an embodiment, the palladium alloy is a palladium-gold alloy, a palladium-silver alloy, a palladium-copper alloy, a palladium-platinum alloy, or combinations thereof, and in another embodiment the platinum alloy is a platinum-gold alloy, a platinum-silver alloy, a platinum-copper alloy or combinations thereof. In an embodiment, a metallic component comprises at least 50 wt. %, preferably at least 70 wt. %, more preferably at least 90 wt. % of the plurality of nanoparticles, where the metallic component consists of one of palladium, platinum, a palladium alloy, a platinum alloy, and combinations thereof. In another embodiment at least 30 wt. % and in a further embodiment at least 50 wt. % of the metallic component consists of palladium, platinum, or combinations thereof. Here, palladium alloy means a stoichiometric or non-stoichiometric solid solution of palladium and an alloying element. In an embodiment, at least 30 wt. % of the palladium alloy consists of palladium, and in a further embodiment at least 50 wt. % of the palladium alloy consists of palladium. Similarly, platinum alloy means a stoichiometric or non-stoichiometric solid solution of platinum and an alloying element. In an embodiment, at least 30 wt. % of the platinum alloy consists of platinum, and in a further embodiment at least 50 wt. % of the platinum alloy consists of platinum. Additionally, the plurality of Pd-based nanoparticles, Pt-based nanoparticles, and combinations thereof have an average nanoparticle diameter of less than about 100 nanometers. In an embodiment, the average nanoparticle diameter is less than 10 nanometers, and in another embodiment, less than 5 nanometers. The average size of the Pd-based and Pt-based nanoparticles may be tailored in order to derive a desired response. For example, relatively large particles approaching 100 nm in diameter may strongly scatter light in the visible range while relatively small particles less than approximately 10-20 nm may not. In some embodiments, nanoparticle diameters less than 10 nanometers may be utilized to provoke absorption based responses to avoid potential interference from background light coupled into the H₂ sensing material. Tailoring the particle size can therefore affect the wavelength and optical response of a metallic nanoparticle extinction, absorption, and scattering cross-section. In an embodiment, the average nanoparticle diameter is greater than about 2 nm. The average nanoparticle diameter may be determined using various methods known in the art for the sizing of nanoparticles, for example, scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM) methods. Preferably, the average particle size is determined through image analysis by capturing a sample of typically at least 100 nanoparticles, more preferably at least 300 nanoparticles. However, as is understood, the method by which an average nanoparticle diameter is determined is not limiting within this method.

It is understood that the nanoparticles of this disclosure are not limited to strictly spherical shapes, and that the plurality of gold nanoparticles may be comprised of shapes such as triangular prisms, disks, shells, wires, rods, and others. When such structures are present, the average particle diameter refers and is equivalent to an equivalent circular diameter (ECD), which connotes the diameter of a circle with area equal to that of the projection of the particle on a plane. See e.g., Xu et al, “Comparison of sizing small particles using different technologies,” Powder Technology 132 (2003).

Additionally, the inert matrix has a bandgap greater than or equal to 5 eV, and has an oxygen ion conductivity less than approximately 10⁻⁷ S/cm at a temperature of 700° C., where the oxygen ion conductivity is either known from compiled sources or determined using techniques known in the art, such as the oxygen permeation method. See e.g. Kagomiya et al., “Oxygen permeation and microstructure of intergrowth perovskite Sr—La—Fe—Co based mixed conductive ceramics,” J. Ceram. Soc. Jpn. 117 (9) (2009); and see Chen et al., “Ionic conductivity of perovskite LaCoO₃ measured by oxygen permeation technique,” J. Appl. Electrochem. 27 (1997), among others. The inert matrix is generally based upon a stoichiometric dielectric material, such as SiO₂, Si₃N₄, or Al₂O₃. In an embodiment, the matrix material comprises an inorganic metal oxide of the formula M_(a)O_(b), where M comprises one or more metals. In some cases, the inert matrix may consist of more complex systems such as MgF₂-doped SiO₂, or mixed SiO₂/Al₂O to tailor the effective refractive indices for optimized sensing response in optical waveguide based sensing applications. Such an inert matrix as defined within this disclosure has limited free carriers, and can be expected to display negligible change in refractive index in response to reducing species that may occur in a monitored stream even at the high temperature conditions of interest relevant for this disclosure. Additionally, the inert matrix acts to mitigate the coarsening of Pd-based or Pt-based nanoparticles which might otherwise occur under the high temperature conditions of this disclosure. In some embodiments the inert matrix may act to prevent oxidation and/or reduction of the Pd-based and/or Pt-based nanoparticles thereby increasing selectivity to H₂ in a broad range of atmospheres. In other embodiments, oxidation and reduction reactions may be permitted and desirable for an optimized sensing response.

The inert matrix supporting the plurality of Pd-based and/or Pt-based nanoparticles is permeable at least to some degree to the incident light at wavelengths corresponding to the wavelength or wavelength range over which the optical signal generates. For example, when optical signals are generated within a light wavelength range from about 300 nm to about 850 nm or a smaller range, the inert matrix is permeable at least to some degree to the incident light at wavelengths from about 300 nm to about 850 nm or within the smaller range. Similarly, when optical signals generate within a light wavelength range from about 1350 nm to about 1750 nm or a smaller range, the inert matrix is permeable at least to some degree to the incident light at wavelengths from about 1350 nm to about 1750 nm or within the smaller range. In an embodiment, the inert matrix is permeable at least to some degree to light at wavelengths over the visible spectrum. Here “visible spectrum” connotes light having wavelengths from about 400 nm to about 750 nm. The optical properties of the inert matrix are such that the inert matrix has a refractive index greater than one. In some embodiments, the inert matrix has an extinction coefficient of less than 10⁻⁵ cm⁻¹ over the wavelength range of interrogation, and in other embodiments, the inert matrix has an extinction coefficient of less than 10⁻³ cm⁻¹ over the wavelength range of interrogation.

As discussed, within hydrogen sensing material 101, the Pd-based nanoparticles, Pt-based nanoparticles, and combinations thereof are dispersed in the inert matrix. As used herein, “dispersed” means that individual nanoparticles comprising the plurality of Pd-based nanoparticles, Pt-based nanoparticles, and combinations thereof in hydrogen sensing material 101 are sufficiently separated such that hydrogen sensing material 101 displays an electrical conductance at least an order of magnitude less than the electrical conductance of a bulk material comprised of the same material as the Pd and/or Pt-based nanoparticles under an equivalent temperature condition, such that the electrical conductance is less than 1/10^(th) of the electrical conductance of the bulk material. Such a condition can be determined using various methods for the evaluation of proximity to a percolation limit in supported nanoparticle systems. See e.g. Trudeau et al., “Competitive transport and percolation in disordered arrays of molecularly linked Au nanoparticles,” J. Chem. Phys., Vol. 117 (2002), among others. Additionally, in an embodiment, an average spacing between individual Pd and/or Pt-based is at least five times greater than the average nanoparticle diameter, where average spacing indicates the average displacement between a given nanoparticle and a nearest nanoparticle neighbor. For the given nanoparticle, the nearest nanoparticle neighbor may be determined through a variety of statistical methods known in the art, such as fixed radius analysis, minimal spanning trees, Voronoi polygons, k-nearest neighbor algorithms, and other established nearest neighbor methodologies. See e.g., Dussert et al., “Minimal spanning tree: A new approach for studying order and disorder,” Phys. Rev. B, 34 (5) (1986), and see Aurenhammer, “Voronoi Diagrams—A Survey of a Fundamental Geometric Data Structure,” ACM Comput. Surv., 23(3) (1991), and see Cover et al., “Nearest Neighbor Pattern Classification,” IEEE T. Inform. Theory 13(1) (1967), among others. The displacement between the given nanoparticle and the nearest nanoparticle neighbor may be determined using techniques such as scanning electron microscopy, atomic force microscopy, and transmission electron microscopy, and the average spacing may be determined as the average value over some statistically significant population of Pd and/or Pt-based nanoparticles, for example at least 100 nanoparticles.

The Pd and/or Pt-based nanoparticles may be dispersed relatively uniformly or non-uniformly with respect to the inert matrix, provided that the nanoparticles are dispersed within the meaning of the definition discussed above. For example, FIG. 9 illustrates hydrogen sensing material 901 in contact with gas stream G, where hydrogen sensing material 901 comprises a plurality of Pd-based nanoparticles, Pt-based nanoparticles, and combinations thereof such as nanoparticle 926 within inert matrix 927, and where hydrogen sensing material 901 is further in contact with a substrate 928. At FIG. 9, the plurality of Pd and/or Pt-based nanoparticles are distributed relatively homogenously through inert matrix 927, such that the relative concentration of Pd and/or Pt-based nanoparticles is substantially similar within inert matrix 927 as displacement occurs in a negative direction of the y-axis illustrated. In an embodiment, a majority of a statistically significant population of Pd-based nanoparticles, Pt-based nanoparticles, and combinations thereof are within inert matrix 927 such that at least some portion of inert matrix 927 is between the majority and gas stream G. Similarly, FIG. 10 illustrates hydrogen sensing material 1001 in contact with gas stream G, where hydrogen sensing material 1001 comprises a plurality of Pd-based nanoparticles, Pt-based nanoparticles, and combinations thereof such as nanoparticle 1026 within inert matrix 1027, and where hydrogen sensing material 1001 is further in contact with a substrate 1028. FIG. 10 depicts the plurality of Pd and/or Pt-based nanoparticles concentrated in a region of inert matrix 1027 such that the relative concentration of nanoparticles increases within inert matrix 1027 as displacement occurs in a negative direction of the y-axis illustrated, and decreases or disappears as displacement occurs in a positive direction of the y-axis illustrated. However, arrangements such as those in FIGS. 9 and 10 are exemplary only, and the relative concentrations of Pd-based nanoparticles in varying regions of the inert matrix are not limiting within this disclosure provided the plurality of Pd-based nanoparticles, Pt-based nanoparticles, and combinations thereof are dispersed within the inert matrix as discussed above.

Additionally, it is understood that when the disclosure describes monitoring an optical signal and thereby evaluating the hydrogen concentration of the gas stream, this includes operations where a shift in the optical signal serves as an indication of a change in the hydrogen concentration of the gas stream. As an example using the results of FIG. 3 for the hydrogen sensing material of Pd/SiO₂, the hydrogen sensing material generates the optical signal 310 at a first magnitude A₁ at an H₂ concentration of 2% and at a second magnitude A₂ at an H₂ concentration of 10%. Here, evaluating the hydrogen concentration of the gas stream may comprise treating the first magnitude A₁ as a measurand and assigning a specific value of H₂ to the measurand such as 2%, or may comprise monitoring the optical signal at one or more wavelengths, observing a shift in the optical signal such as from A₁ to A₂, and treating the observed shift as indicative of a change in H₂. Here, a “shift in the optical signal” means a variation between an initial optical signal and a subsequent optical signal at one or more wavelengths, where the initial optical signal is generated at a first time and the subsequent optical signal is generated at a second time, and where both the initial optical signal and the subsequent optical signal are generated by illuminating the hydrogen sensing material with the light source emitting the incident light, collecting the exiting light, and comparing the incident light and the exiting light using optical spectroscopy. The shift in the optical signal may be recognized by detecting a variation between optical signals at any monitored wavelength or by variations at multiple wavelengths over a band of wavelengths. For example, the variation may be detected by monitoring a transmittance at a specific wavelength, the specific wavelength of an optical signal edge within a specified wave length range, the wavelength of an optical signal local maxima, a variation in the optical signal breadth, a variation in the optical signal amplitude, a variation in the optical signal full width at half maximum (FWHM), or any other techniques which may serve to indicate a variation between the initial optical signal and a subsequent optical signal. In an embodiment, the shift in the optical signal means a variation of at least 0.1% between an initial time-averaged optical signal and a subsequent time-averaged optical signal in transmittance, absorptance, or reflectance at a specific wavelength.

In a particular embodiment, the hydrogen sensing material is employed in an instrument such as that illustrated at FIG. 11. In this embodiment, the hydrogen sensing material of this disclosure comprises a sensing head 1170 in contact with a gas stream G. An interrogator 1171 illuminates the hydrogen sensing material comprising sensing head 1170 with incident light and gathers exiting light. Interrogator 1171 compares the incident light and the exiting light and generates a measurand, where the measurand is proportional to the optical signal as defined herein. Such interrogators for use in optical systems are known in the art. See e.g., Lee et al., “Review of the present status of optical fiber sensors,” Optical Fiber Technology 9 (2003), and associated references. Interrogator 1171 is in data communication with meter 1172 which provides an indication of the magnitude of the measurand generated and communicated by interrogator 1171. In this embodiment, the steps of illuminating the hydrogen sensing material, collecting exiting light, and monitoring an optical signal based on a comparison of the incident light and the exiting light is conducted by interrogator 1171, and monitoring the optical signal is conducted through observation of meter 1172. An indication of the H₂ concentration of gas stream G is provided by comparison of the observed meter reading and a reference meter reading, where the reference meter reading results from a reference measurand generated under reference conditions.

The hydrogen sensing material utilized in the method of this disclosure may be prepared using means known in the art for the production of gold nanoparticles dispersed in a supporting matrix, including sol-gel wet chemistry based techniques, impregnation techniques, implantation techniques, sputtering techniques, and others. See e.g. Ohodnicki et al., “Pd/SiO₂ and AuPd/SiO₂nanocomposite-based optical fiber sensors for H₂ sensing applications,” Sensors and Actuators B 214 (2015). Generally, rigorous calcination schedules will improve the temperature stability of the resulting material under the reducing conditions of this disclosure. A high calcination temperature and long calcination time may ensure that the resulting hydrogen sensing material is properly aged, so that any optical property changes occurring in the material during sensing operations at the higher temperatures of this disclosure can be attributed to H₂ concentration of the monitored stream. However, the specific manner in which the hydrogen sensing material of this disclosure is prepared is not limiting within this disclosure, provided that the hydrogen sensing material is comprised of Pd-based nanoparticles dispersed in an inert matrix as defined herein.

Thus, provided here is a method for H₂ sensing in a gas stream which utilizes an optical signal generated by a hydrogen sensing material. The hydrogen sensing material is comprised of a plurality of Pd-based nanoparticles, Pt-based nanoparticles, and combinations thereof dispersed in a wide bandgap matrix with a low oxygen ion conductivity. The method disclosed offers significant advantage over materials typically utilized for H₂ sensing, including enhanced thermal stability, relative insensitivity to reducing gases beyond H₂, increased stability of nanoparticle diameter, tunability of effective refractive indices for compatibility with optical waveguide based sensors, among others.

It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention and it is not intended to be exhaustive or limit the invention to the precise form disclosed. Numerous modifications and alternative arrangements may be devised by those skilled in the art in light of the above teachings without departing from the spirit and scope of the present invention. It is intended that the scope of the invention be defined by the claims appended hereto.

In addition, the previously described versions of the present invention have many advantages, including but not limited to those described above. However, the invention does not require that all advantages and aspects be incorporated into every embodiment of the present invention.

All publications and patent documents cited in this application are incorporated by reference in their entirety for all purposes to the same extent as if each individual publication or patent document were so individually denoted. 

What is claimed is:
 1. A method of evaluating a hydrogen concentration of a gas stream comprising: placing a hydrogen sensing material in the gas stream, where the hydrogen sensing material comprises, an inert matrix, where the inert matrix is stable at the gas stream temperature, and where the inert matrix is optically transparent over a light wavelength range, and where the inert matrix has a bandgap greater than or equal to 5 eV and has an oxygen ion conductivity of less than 10⁻⁷ S/cm at a temperature of 700° C., a plurality of nanoparticles dispersed in the inert matrix, where a metallic component comprises at least 50 wt. % of the plurality of nanoparticles, where the metallic component consists of one of palladium, platinum, a palladium alloy, a platinum alloy, and combinations thereof, and where the plurality of nanoparticles have an average nanoparticle diameter of less than about 10 nanometers; illuminating the hydrogen sensing material with a light source emitting incident light; collecting exiting light, where the exiting light is light that originates at the light source and is transmitted, reflected, scattered or a combination thereof by the hydrogen sensing material; monitoring an optical signal based on a comparison of the incident light and the exiting light using optical spectroscopy, thereby evaluating the hydrogen concentration of the gas stream.
 2. The method of claim 1 where the metallic component comprises palladium, and where an inorganic metal oxide having a formula M_(a)O_(b) comprises at least 50 wt. % of the inert matrix.
 3. The method of claim 1 where the metallic component comprises platinum, and where an inorganic metal oxide having a formula M_(a)O_(b) comprises at least 50 wt. % of the inert matrix.
 4. The method of claim 1 further comprising: providing a waveguide comprised of a core material; placing the hydrogen sensing material in contact with the core material; emitting the incident light from the light source into the core material and generating an evanescent wave; and illuminating the hydrogen sensing material with the evanescent wave, thereby illuminating the hydrogen sensing material with the light source emitting the incident light. 